I need to implement a robotic gripper design for my project. I would like one that functions more like a clamping mechanism in which theupper and lower "fingers" move toward each other in a linear direction as opposed to gripper fingers that rotate inward in a circular manner. I need it to open at least 3 inches, and it needs to be not very expensive since I need three of these grippers. Does such a product exist? If not, I'll need to design my own and would need to know what kind of actuator to use: rack and pinion, screwdriver, or linear stepper motor?